Refactor README layout

[0xroylee]

What Changed

• Reworked the root README.md into a product-first overview with a stronger opening pitch, fit/benefits section, problems-solved framing, under-the-hood architecture, quickstart, and docs navigation.
• Updated packages/cli/README.md to keep the package README focused on CLI usage while aligning its opening with the root README and fixing related relative doc links.
• Preserved the existing install/onboard/daemon flow and command references.

Why

• The repo’s README needed a clearer top-level narrative that reads more like a product page and less like a command dump.
• The CLI README and root README were overlapping in audience and needed a more consistent hierarchy.

How to Test

• bun run typecheck passed.
• bun test passed.
• git diff --check on the touched README files passed.
• README content scans confirmed ASCII-only text and correct relative links.
• bun run check still fails because of unrelated Biome issues in packages/web/src/components/chat-room/chat-room-settings-sidebar.tsx and packages/web/src/components/chat-room/chat-room-sidebar.tsx.

Additional Notes

• This change is documentation-only.
• The repository-wide lint gate remains blocked by pre-existing web formatting/import-order issues outside the README scope.